The present invention relates to safety relief valves for use on a pressurized system such as pressure vessel or a flow line, especially snap-type safety relief valves having a consistent low blow-down value, springs for safety relief valves and methods of manufacturing safety relief valves.
Spring operated relief valves are used to protect pressurized systems from pressures that exceed their maximum allowable working pressure or any pressures that the user specifies. Most spring operated relief valves use an externally adjustable coil springs that, when compressed, applies the force to keep the valve closed. The spring, which is externally adjustable by the means of a threaded adjustment screw, can be set to allow the valves to operate at a wide range of pressures. The pressure that an individual valve is set to open at is called the set pressure. When this set pressure is reached the valves opens and relieves the excess pressure. The valve then closes when the system pressure has dropped to a reduced level.
Snap-type safety relief valves have the advantage of responding very quickly to pressure changes in pressurized systems to which they are attached. Snap-type safety relief valves move to a fully open position almost immediately after the pressure within the pressure vessel rises above the user-determined set pressure. This allows excess gas pressure to escape quickly. Then, when sufficient pressure has escaped, snap-type safety relief valves quickly and crisply move back to a closed position. For an example of a snap-type safety relief valve, see U.S. Pat. No. 3,664,362, which is herein incorporated by reference in its entirety.
A “blow-down value” is the percentage difference between the user-determined set pressure and the pressure in the pressure vessel or flow line when the snap-type safety relief valve snaps closed. For example, if a user sets the set pressure at 100 psi, and valve stays open as gas escapes out of the snap-type safety relief valve until the pressure in the pressure vessel or flow line is 70 psi, then this snap-type safety relief valve has a blow-down value of 30%. For an example of a snap-type safety relief valve having a standard blow-down value, see U.S. Pat. No. 4,799,506, which is herein incorporated by reference in its entirety. Low blow-down valves have a blow-down value of about 15% or less, preferably about 10% or less. A particularly preferred valve will have a blow-down of 5% to 10% of set pressure. If the set pressure were, for example 100 psi, the reseat pressure would fall in the range of 90 psi to 95 psi for such a preferred valve. Low blow-down valves are desirable because they can minimize the amount of gas that is lost from the pressurized system into the atmosphere during venting, thereby addressing environmental concerns.
Existing low blow-down snap-type safety relief valves do, however, have some problems. One problem is that the blow-down values of the valves are affected by built-up downstream back pressures. The term “built-up downstream back pressures” is well understood in the art and documented in the American Petroleum Institute Recommended Practice 520. The length of outlet piping and the number of elbows that are attached to the outlet of the snap-type safety relief valves contributes to built-up downstream back pressures. Generally, the longer the outlet piping and the greater the number of elbows in the outlet piping, the more built-up downstream back pressure is are created.
Built-up downstream back pressures affect the blow-down value of typical snap-type safety relief valves. For example, a manufacturer may sell a snap-type safety relief valve with a blow-down value of 10% that is recommended to be used with 10 feet of outlet piping. At an installation site, the installer may disregard the manufacturer's recommendations and use 20 feet of outlet piping. In such a case, when the valve is in use, it will experience greater built-up downstream back pressures than the manufacturer designed for. The additional built-up downstream back pressures counteract forces that keep the valve open, and may cause the snap-type safety relief valve to close prematurely. If this occurs while the pressurized system still needs to vent, the snap-type safety relief valve would open again. The valve may then open and close in rapid succession, which is a phenomenon known as chatter. Chatter can shorten the life of a snap-type safety relief valve.
Another problem with existing low blow-down snap-type safety relief valves is that they tend to have a sliding-fit piston/sleeve design. See, for example, the snap-type safety relief valve described in U.S. Pat. No. 3,411,530, which is herein incorporated by reference in its entirety. In these designs, when the piston is raised so that gas may escape, a portion of the sleeve may obstruct the flow path as fluid flows through holes in the sleeve. As fluid escapes, foreign particles tend to accumulate between the sliding surfaces, causing additional friction between the sliding-fit parts. This build-up of foreign particles can cause freeze-up of the piston. This can affect the amount of pressure necessary to open the valve, and it can affect the blow down value of the valve, making the valve's performance less predictable. Also, contaminated gas can cause the valve to malfunction.
In addition to the above-mentioned problems, low blow-down snap-type safety relief valves can vary in quality in a number of ways. All snap-type safety relief valves each have a flow coefficient, which represents how unobstructed gas flows through the valve when it is fully open. Higher flow coefficients are considered to be better. Also, different snap-type safety relief valves vary in their ability to maintain their blow-down value, their performance reliability, their durability, their cost to manufacture, and their ease of use.
It is well known in relief valve art that for any given valve design, such as that disclosed in parent application Ser. No. 09/885,293, that as set pressures increase, the slope of the “Force-lift Curve” becomes steeper, i.e., the ratio of force to lift becomes larger. A detailed discussion of the “Force-lift Curve” is provided in U.S. Pat. No. 4,799,506, particularly in reference to FIG. 6 of that patent.
To obtain consistently low blow-downs over a broad range of set pressures for a given valve model, say 80 psi to 1500 psi, a series of springs, typically numbering from 15-20, is required.
A single spring in a relief valve will produce a range of operation for one size of valve, which is dependent on the rate of the spring. As the rate of the coil spring increases, the blow-down will also decrease. For a standard relief valve this range is quite large, achieving 20% blow-down at a set pressure and then 40% blow-down at some higher set pressure. In this type of valve line, the next higher pressure valve would be made with a higher rate spring that would achieve a blow-down of 20% at the same pressure that the previous valve produced a 40% blow-down. This is then repeated throughout the entire valve line so that when the individual valves are put together, they will cover a range of, for example, 15-2500 (psig). Because of standard spring rate tolerance these valves are designed with overlapping pressure ranges, which results in a slightly reduced operating range. This ensures that, even though springs rate tolerances drift, each valve will function properly over its intended operating range. Standard blow-down relief valves have very large spring ranges so that this overlap only necessitates the availability of a few sizes of springs in the inventory used to make the valves. Low blow-down valves, on the other hand, have operating ranges so short that any small over lap will greatly increase the number of springs needed for the valve line. In fact the overlap needed for a ±7% tolerance spring is so large that a 5%-10% blow-down valve line is not practical. A spring with a rate tolerance of ±5% would be about the maximum rate drift for this type of valve line. A rate tolerance of no greater than ±2%, ideally no greater than ±1%, would allow for a practical amount of springs in this line. This is an unrealistic tolerance due to spring manufacturing limitations and cost.
For example the following springs might be typical for a 0.788 in. diameter orifice safety relief valve, where increased wire sizes are used to keep the maximum allowable stress below an acceptable design limit. Lift values in ASME code-designed valves are constant over the set pressure range of any given orifice size.
Set Point RanqeSpring RateWire Size 77.5-97.5 psi 80 lbs/in..125 in.  97.5-120 psi 104 lbs/in..125 in. 120-145 psi 128 lbs/in..136 in. 145-173 psi 154 lbs/in..136 in. 173-203 psi 182 lbs/in..148 in. 203-240 psi 214 lbs/in..168 in. 240-234 psi 248 lbs/in..177 in. 337-403 psi 289 lbs/in..177 in. 403-485 psi 335 lbs/in..177 in. 485-585 psi 388 lbs/in..187 in. 585-720 psi 450 lbs/in..203 in. 720-840 psi 610 lbs/in..203 in. 890-1120 psi  714 lbs/in..225 in.1120-1420 psi   838 lbs/in..225 in.1420-1840 psi   990 lbs/in..250 in.1840-2420 psi  1175 lbs/in..250 in.
In order to insure that spring ranges do not over lap, and meet a specification of a 5% to 10% blow-down for each spring, very precise control of the rate of any given spring is required. Again refer to FIG. 6 of U.S. Pat. No. 4,799,506.
Helical or coil springs are one of the most common types of springs and are produced by the millions each year. Coil compression springs produce resistance to a compressive force that is applied through its central axis. These springs, when coiled with a constant diameter, produce a resistive force that is directly proportional to the spring's displacement. This property makes coil springs very predictable and easy to work with. The term rate, which is represented by R, describes the spring's ability to resist a given force. This can be shown mathematically by the equation: R=F/D where F is force acting on the spring and D is the displacement due to that force. For springs that have a constant diameter and wire size, the rate is constant or linear, which mean that at any point of displacement the spring's rate stays the same. Constant rate coil springs are designed to a desired rate, which is dependent on four factors: wire size and modulus, the spring's mean diameter, and number of coils. Deviations and inconsistencies in these four factors make spring production not an exact science. Spring tolerance will vary form one spring manufacturer to another, but in general as design tolerances decrease so does the cost of producing the springs.
Most coil springs are wound on a highly efficiently machine that can produce standard springs very cheaply. For most spring manufactures, +10% deviations in spring rates is about average, and a rate around ±7% is considered about as tight as possible. Other types of coil spring manufacturing are available, such as springs that are machined for a blank. These machined springs can be produced to tighter tolerances than conventional springs, but are extremely expensive compared to wound coil springs. Because of this high rate tolerance, most spring applications have allowances for rate deviations, but in the safety relief valve industry, these deviations can be problematic for low blow-down valves. Spring operated relief valves are mass-produced with standard springs and, because of design allowances, work fine. Low blow-down relief valves with short spring ranges, on the other hand, need as tight of a spring rate tolerance as possible.
One approach to obtaining a rate tolerance of ±2% would be to test all springs from a good commercial spring vendor, and discard those that have rates outside of the ±2% tolerance. This, of course, is very wasteful. In order to realize a 5% to 10% blow-down, as many as 40 different springs would have to be designed and purchased to cover the set pressure range for a given orifice size. The rate for each spring would then be recorded and sorted. To fill an order for any set pressure over the range of 80 psi to 1800 psi, a specific spring would then be selected with the rate required to produce a relief valve with a blow-down in the 5% to 10% range. While the procedure could be effective, it would be inefficient from a spring inventory and manufacturing perspective.
Thus there is a need for a way to modify springs for use in low blow-down valves so that they will have a rate within a ±2% tolerance. Also, a low blow-down valve design is needed to accommodate such modified springs.